Blindness is still nowadays difficult to cure. Technologies such as speech synthesizers, 3D tactile displays and dedicated scanners improve the quality of life of blind persons by allowing them to read text and manipulate money. However, for seeing., the situation is still the same as a hundred, or even a thousand years ago.
Since electrical stimulation techniques are applied in many circumstances to enhance or restore organ function, a few research teams are working on the recuperation of a limited but functional vision to totally blind persons. A functional vision means that the person will be able to do, without assistance, most of the tasks being part of every day life. It will be limited since no system in the near future will be able to replace the natural vision system with the same accuracy. The required resolution and data processing capabilities are simply too large.
A person is considered legally blind if a visual dysfunction is present and sufficient to greatly affect his everyday life. The medical criterions vary from one country to another but, in general, those who are considered legally blind include a specific group of totally blind persons. This means that they do not see anything and live in a world of complete darkness. The causes of blindness are numerous. Some causes originate in the eye and others are related to the visual pathways.
The history of human visual stimulation began in 1960 when it was found that when a specific part of the human brain was stimulated with an electrical current, a fixed light spot appeared in the visual field of the patient. The part of the brain was later identified as the visual cortex and the light spots are called phosphenes. In 1968, results of clinical experiments related to visual stimulation were first published. The experiments were done with different voltage sources and spacing between electrodes through an array of 81 platinum electrodes. In all cases, the electrical stimulation was done on the surface of the visual cortex. As research progressed, notable discoveries were made and can be summarized as follows: current based intracortical stimulation leads to a significant current reduction, more stable phosphenes and a phosphene intensity that is proportional to the current. To accomplish the visual stimulation, two ma in steps are necessary. The first is to acquire a real life scene and generate stimulation information, or stimulation command words. The second is to inject the proper electrical current to do the stimulation according to the command words.
There are at least three undertaken research activities intended to create adequate vision using electrical stimulation. Each one has its own distinctive characteristics, which are the following:
1) Retina stimulation, where an electrode array is inserted into the light sensitive retina. The advantage of this method is to use most of the natural visual pathway. It is an advantage but also an inconvenience since the visual pathway must be intact and functioning properly. Some of the best challenges of this method of stimulation are mechanical. Since the electrode array is located on the retina, it will be subjected to the very large angular accelerations of the eye. The electrode array must be secured in place very firmly to avoid damaging the retina. Furthermore, to have a good contact with the retina, the electrode array must not be planar but must match the spherical nature of the eye. This approach seems to be dedicated to vision enhancement because the visual pathway is intact. For example, it would be ideal for patients losing the sensitivity of their peripheral vision.
2) Cortical stimulation, where the electrode array is inserted into the brain visual cortex. This method is also dedicated to totally blind persons. Its only requirement is that the visual cortex be intact, which seems to be the case more than 90% of the time. Research is under progress to determine long term stimulation effects on the brain and cell damage due to a high density of electrodes, but the preliminary results are encouraging. A critical step to this method is the insertion of the electrode array into the visual cortex. The current approach suggests a pneumatic system.
3) Optical nerve stimulation is a new stimulation strategy recently introduced. Obviously, the visual pathway must be intact from the optic nerve to the visual cortex. The exact nature of the signals carried by the optic nerve is not thoroughly known and more research is needed before feasibility can be demonstrated.
Known in the art are U.S. Pat. No. 4,551,149 (Sciarra); U.S. Pat. No. 4,628,933 (Michelson); U.S. Pat. No. 5,159,927 (Schmid); U.S. Pat. No. 5,215,088 (Normann et al.); U.S. Pat. No. 5,324,315 (Grevious); U.S. Pat. No. 5,324,316 (Schulman et al.); U.S. Pat. No. 5,876,425 (Gord et al.); U.S. Pat. No. 5,800,535 (Howard, III); U.S. Pat. No. 5,807,397 (Barreras); U.S. Pat. No. 5,873,901 (Wu et al.); U.S. Pat. No. 5,935,155 (Humayun et al.); UK patent application GB 2,016,276 assigned to W H Ross Foundation (Scotland) for Research into Blindness and published on Sep. 26, 1979; and Canadian patent No. 908,750 (Brindley et al.) issued on Aug. 29, 1972, depicting the state of the art.
The above patent documents show that various implants have been designed, at least on a theoretical basis. However, many problems arise when the time comes to put them into practice. Difficulties in the production of electronic implants lay for example in the integration of the various required functions and the miniaturisation of the whole system. The existing implants exhibit high power consumption as they are built using separate electronic modules that further take significant space. The RF part, operating at high speed, is generally made with discrete electronic components due to the electromagnetic interferences generated by this part; it is thus not integrated with the rest of the implant circuit, which would otherwise allow a reduction of the dimensions and the power consumption of the implant. Since an implantable system with discrete components has a high power rating, its power supply by an inductive link is thus hardly practicable. A few designs group the electronics and the electrodes on the same silicon slice. This method facilitates achievement of a vector of a few electrodes, but its application to a large number of electrodes in an array format remains to be proven. The efficiency of an inductive coupling to supply the implantable part of the system is very low because the majority of the currently used techniques are based on ASK modulation. This low efficiency prevents the integration of all the desired functions in the same implant when discrete component designs are used.
The implants used for electric stimulation purposes are thus far unable to monitor changes on the electrode-tissue interface. Such a monitoring function is however highly desired to monitor and follow the evolution of the milieu in contact with the implanted system. The majority of the existing systems are unable to process a large number of inputs and outputs (many tens and hundreds); they are mostly designed for a few stimulation channels only, e.g. for a 10×10 array of electrodes. Furthermore, the assembly of implant electronics with an electrode array having a large number of electrodes in a surface having reduced dimensions has so far not received much attention in the art, as for some other aspects related to implants and implant systems.